It's also an indication that he stands firm on his own views and could act as a moderating force within the Trump administration, as some lawmakers and national security experts hope.

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Mattis will also list strengthening alliances as one of his priorities, along with strengthening military readiness and reforming the Pentagon.

“If you confirm me, my watchwords will be solvency and security in providing for the protection of our people and the survival of our freedoms,” he will say.

“My priorities as secretary of Defense will be to strengthen military readiness, strengthen our alliances in league with our diplomatic partners, and bring business reforms to the Department of Defense by instilling budget discipline and holding our leaders accountable.”

Mattis retired from the military in 2013 and entered the business world as a board member of General Dynamics and Silicon Valley startup Theranos. He resigned from Theranos earlier this month and will step down from General Dynamics if confirmed for the position.

Mattis will need a waiver from Congress exempting him from a law requiring a Defense secretary to be out of uniform for at least seven years.

Some senators, however, are concerned about the waiver eroding the principle of civilian control of the military.

Mattis in his remarks will call civilian control a “fundamental tenet of the American military tradition” and speak about its importance.

"If the Senate consents and if the full Congress passes an exception to the seven-year requirement, I will provide strong civilian leadership of military plans and decisions,” he will say.

"I recognize under the Constitution it is the Congress that raises, sustains, and supports our armed forces through annual authorizations and appropriations,” he will add.